Set a function that is called when the signal event source is destroyed.

Detailed Description

UNIX signal support for main loops.

In contrast to other main loop event sources such as timer and IO events, UNIX signal support requires modification of the global process environment. Due to this the generic main loop abstraction layer as defined in mainloop-api.h doesn't have direct support for UNIX signals. However, you may hook signal support into an abstract main loop via the routines defined herein.